1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method of performing a handover between heterogeneous networks and user equipment apparatus for the same.
2. Discussion of the Related Art
First of all, handover means a function of maintaining a call status continuously in a manner that a user equipment in a call status is automatically tuned to a new call channel of a neighbor base station in the course of moving away from a corresponding base station service area (e.g., a cell boundary) into a neighbor base station service area. Since a call cutoff time in the course of switching a call channel automatically amounts to 15 ms or less, when message exchange is performed between a base station and a user equipment during such a short time, it may be difficult for a calling subscriber to detect an instant call cutoff state. As the number of user equipments is rapidly increasing due to the ongoing development of wireless communication technology, heterogeneous networks of different wireless communication schemes are expanding. Therefore, many ongoing efforts are made to research and develop the technology of handover between heterogeneous networks or media.
Handover methods may be categorized into hard handover (HHO) schemes and soft handover schemes. The soft handover schemes may be categorized into macro diversity handover (MDHO) scheme and fast base station switching (FBSS) scheme. Yet, since the soft handover scheme has a problem of excessive overhead, many ongoing efforts are made to research and develop the hard handover scheme.
Recently, the wireless mobile communication fields consistently keep being evolved in aspect of fast data transmission and reception as well as voice call. And, ongoing attentions are paid to the 4th generation mobile communication technology, e.g., LTE (long term evolution) wireless communication system. Yet, in a current situation that the 4th generation communication network and the commercialized 3rd generation communication network coexist, a mobile communication terminal or a mobile communication data card should include the 3G mobile communication technology, which is already commercialized and being used globally, as well as the 4th generation mobile communication technology. Therefore, in order to support both of the next generation mobile communication technology and the current generation mobile communication technology, a mobile terminal (hereinafter called a dual mode terminal) equipped with a dual modem processor or a data card type device (hereinafter called a dual mode terminal) equipped with a dual modem processor is required.
The dual mode terminal is equipped with two kinds of modems differing from each other in communication scheme and supports the wireless communications using the two kinds of the modems, respectively. And, the dual mode terminal is frequently used in an area in which heterogeneous communication networks coexist. For example of a representative dual mode terminal, attention is paid to a device capable of both LTE (long term evolution) wireless communication and eHRPD (enhanced high-rate packet data) wireless communication.
However, no efforts have been made to research and develop an efficient handover scheme of a terminal (or a user equipment) operating in dual mode.